Magnetic Resonance Imaging (MRI) is an imaging technique based in part on the absorption and emission of energy in the radio frequency range. To obtain the necessary magnetic resonance (MR) images, a patient (or other target) is placed in a magnetic resonance scanner. The scanner provides a magnetic field that causes target atoms to align with the magnetic field. The scanner also includes coils that apply a transverse magnetic field. Radio-frequency (RF) pulses are emitted by the coils, causing the target atoms to absorb energy. In response to the RF pulses, photons are emitted by the target atoms and detected as signals in receiver coils.
Radio frequency pulses delivered by the coils are precisely calibrated to deliver the appropriate amount of power to maximize tissue contrast. Emitting RF pulses with too much power, however, may result in a host of undesired effects, such as tissue or cellular damage. Specifically, high-power RF-energy levels may cause an overheating of tissues inside a patient, as well as the absorption of harmful levels of radiation. Absorption of electromagnetic energy by the tissue is described in terms of Specific Absorption Rate (SAR), which is expressed in Watts/kg. SAR in MRI is a function of many variables including pulse sequence and coil parameters and the weight of the region exposed. In the United States, for example, the recommended SAR level for head imaging is 8 Watts/kg. Consequently, government restrictions are in place limiting how much power may be delivered into a patient's tissues during RF pulses.
Several techniques, shown to be useful for pathology diagnosis, unfortunately, require substantial RF power and, consequently, SAR levels may be unacceptable for clinical use. Unfortunately, reducing the amount of power delivered to patients to meet acceptable SAR levels produces clinical images with artifacts and reduced tissue contrast. Imperfections in magnetic fields of a scanner interfere with the scanner's ability to produce high-contrast/artifact free images; especially at low RF-power levels.
One conventional technique used to compensate for these imperfections involves injecting intravenous-contrast agents into patients to delineate areas of interest. These contrast agents, however, are often contraindicated for certain patients, and can produce undesirable side effects, including the chance for an anaphylactoid reaction, among other serious risks.
Thus, presently, there is no satisfactory way to produce high-contrast-clinical images free of artifacts, within acceptable SAR levels.